Re: Increase in max_connections

Thanks Bruce. Really interesting, but, I show zone reclaim is already
turned off on our system.

root(at)site-db01b:~ # numactl --hardware
available: 2 nodes (0-1)
node 0 cpus: 0 1 2 3 4 5 12 13 14 15 16 17
node 0 size: 393181 MB
node 0 free: 467 MB
node 1 cpus: 6 7 8 9 10 11 18 19 20 21 22 23
node 1 size: 393215 MB
node 1 free: 319 MB
node distances:
node 0 1
0: 10 20
1: 20 10

root(at)site-db01b:~ # cat /proc/sys/vm/zone_reclaim_mode
0

Thanks,
Karthik

On 3/26/14 1:54 PM, "Bruce Momjian" <bruce(at)momjian(dot)us> wrote:

>On Wed, Mar 26, 2014 at 08:22:01PM +0000, Anand Kumar, Karthik wrote:
>> Looking a little deeper, I saw signs of memory being heavily fragmented:
>>
>> root(at)site-db01b:/var/log # cat /proc/buddyinfo
>> Node 0, zone DMA 1 1 2 2 2 1 0 0 1 1 3
>> Node 0, zone DMA32 8 7 8 7 10 8 7 11 9 5 92
>> Node 0, zone Normal 13069 0 0 0 0 0 0 0 0 0 1
>> Node 1, zone Normal 652315 36885 1168 0 1 1 0 1 1 1 0
>>
>>
>> Node 0 has 13069 4k blocks, and zero 8k blocks available to use
>> Which is likely what caused the problem, I'd think.
>>
>> A little while later though, buddyinfo changed and suddenly there was a
>> lot more memory in 8k blocks.
>>
>> root(at)site-db01b:/proc # cat /proc/buddyinfo
>> Node 0, zone DMA 1 1 2 2 2 1 0
>> 0 1 1 3
>> Node 0, zone DMA32 8 7 8 7 10 8 7
>> 11 9 5 92
>> Node 0, zone Normal 9645 5495 1115 0 0 0 0
>> 0 0 0 1
>> Node 1, zone Normal 409734 10953 1 0 1 1 0
>> 1 1 1 0
>>
>> (Note the change in the Node 0 line, 8k blocks went up from 0 to 5495)
>>
>> Anyone have any idea why memory was so fragmented, and what causes
>>memory
>> to be defragged? Is it something postgres does? Are there any kernel
>> specific settings that control it?
>
>If I had to take a guess, it is zone_reclaim; see:
>
> http://frosty-postgres.blogspot.com/2012/08/postgresql-numa-and-zone-recl
>aim-mode.html
>
>The fix is this sysctl:
>
> vm.zone_reclaim_mode = 0
>
>--
> Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
> EnterpriseDB http://enterprisedb.com
>
> + Everyone has their own god. +